Developed in the 1930s for application in paper and pulp manufacture, duplex alloys are based around a 22% Cr composition and the mixed austenitic-ferritic microstructure that delivers desirable mechanical properties. Today, these stainless steels account for less than 80% of the consumption of duplex, super duplex and hyper duplex grades.
Compared with the generic 304/316 austenitic stainless steel mixture, duplex stainless steel grades will typically have twice the strength and provide a significant uplift in corrosion resistance. This makes them an excellent choice for the engineer looking for a material suitable for an application in a corrosive environment.
Increasing the chromium content of stainless steel will increase its pitting corrosion resistance. However, the Pitting Resistance Equivalent Number (PREN) that infers an alloy’s resistance to pitting corrosion also includes a number of other elements in its formula. This subtlety can be used to explain how the difference between UNS S31803 and UNS S32205 developed and whether it matters.
Following the development of duplex stainless steels, their initial specification was captured as UNS S31803. However, a number of the leading manufacturers were consistently producing this grade to the upper end of the allowable specification. This reflected their desire to maximise the corrosion performance of the alloy, aided by the development of the AOD steelmaking process which allowed tighter control of composition. In addition, it also allowed the level of nitrogen additions to be influenced, rather than just present as a background element.
Therefore, the highest performing duplex grade sought to maximise the levels of chromium (Cr), molybdenum (Mo) and nitrogen (N). The difference between a duplex alloy whose composition meets the bottom of the specification, versus one that hits the top of the specification can be several points based upon the formula PREN = %Cr + 3.3 %Mo + 16 % N.
In order to differentiate the duplex stainless steel produced at the top end of the composition range, a further specification was introduced, namely UNS S32205. Duplex stainless steel made to the S32205 (F60) caption will fully meet the S31803 (F51) caption, whereas the reverse is not true. Therefore S32205 can be dual-certified as S31803.
Langley Alloys stock a comprehensive range of duplex stainless steel as the preferred distribution partner of Sandvik. We stock S32205 in sizes from 5/8″ up to 18″ diameter in round bar, with most of our stock being in the Sanmac® 2205 grade, which adds ‘enhanced machinability as standard’ to the other properties.
Sanmac is a registered trademark of Sandvik Intellectual Property AB.